Editing of list could be done by reusing the same behaviour as in the main code list : double click (or right click) on row to edit either value or label.

I guess this might be some tough dev, since you should need to redefine your data types to be able to link them to a list and add new screens to manage the lists, and keep compatibility with previous save formats. List could (maybe) be bound to only integer values (ie byte, 2 bytes, 4bytes and 8bytes types).

well, the value type won't be a problem as it'll be bound by the value type of the address itself, and the list will be in text and parsed when selected as if you just typed it in

Only problem might be the adding and maintaining of the list

I myself would prefer something like this:
a new rightclick menu option saying : "Attach list to address" when clicked opens a filepicker allowing you to select the choice list
But that skips th maintaining and sorting of the list, which you will then have to do in your own texteditor like notepad.

I see at least 2 problems with having the list in an external file. When you reopen CE, you'll need to repoint to the file or hope it hasn't moved. You'll also need to have a way to reload the file on demand in order to reflect changes. Otherwise I see no major problem in having the list in a simple flat/text file.